BICMOS integrated circuit technology combines bipolar and CMOS transistors in the same integrated circuit. The bipolar transistors have a higher power rating than typical CMOS transistors, which is useful for some circuit applications.
Without limiting the scope of the invention, this background information is provided in the context of a specific problem to which the invention can be applied: fabricating a BICMOS integrated circuit with bipolar transistors having relatively higher collector-emitter, collector-base, and collector substrate breakdown voltages than the normal BICMOS bipolar transistors. The two types of bipolar transistors will be designated as HV (high voltage) and LV (low voltage).
To date, high-performance BICMOS technology has been directed toward conventional 5V digital applications. However, certain applications--such as high voltage analog and high power--require bipolar transistors with a relatively higher breakdown voltage than available using conventional BICMOS fabrication techniques.
In conventional BICMOS fabrication, assuming a P-type substrate, bipolar transistors are fabricated in an N- epitaxial layer with a relatively low dopant concentration formed over a buried N+ region of the substrate that has been implanted with a relatively high dopant concentration. P+ intrinsic/extrinsic base regions are formed in the surface of the N- epitaxial layer, and the emitter is formed by N-doped polysilicon deposited over the base regions--the collector is formed by the underlying N- epitaxial and buried N+ regions.
The avalanche breakdown voltages for the base-collector junction and the collector-substrate junction are inversely proportional to the dopant concentration in the buried N+ collector region. Thus, the relatively high dopant concentration of that region results in a relatively low junction breakdown voltages--typically on the order of 7-10 volts for the collector-emitter, 15-20 for the collector-base, and 25-35 for the collector-substrate. Nevertheless, the relatively high dopant concentration is used to reduce collector resistance and enhance performance.
An additional factor that limits junction breakdown voltages for bipolar transistors is the profile of the base regions. In conventional bipolar fabrication, the base regions have a relatively shallow profile to decrease the emitter-base transit time, thereby increasing the transistor switching speed. However, this base profile enhances base-junction curvature effects as biases are increased for higher voltage applications, which increases electric fields to the point where premature breakdown can occur.
Accordingly, a need exists for a BICMOS fabrication technique that permits the fabrication of HV and LV bipolar transistors on the same BICMOS integrated circuit.